Power cables, particularly power cables insulated with cross-linked polyethylene layer, have heretofore had semiconductive layers formed on each inside and outside the insulating layer for the purpose of moderating electric field. In the sense of avoiding corona discharge, these semiconductive layers are required to intimately contact or bond to the insulating coating without allowing occurrence of any gap. From this point of view, extrusion type semiconductive layers are predominant at present.
The properties required in the semiconductive layers are stable electric conductivity against shear and temperature during processing and also heat cycle, resistance to heat over long periods of time (resistance to thermal aging), extrusion moldability, smoothness of extruded surface, intimate adhesiveness with the insulating layer, resistance to heat distorsion, and thermal stability at high temperatures (generation of corrosive thermal decomposition gas during the cross-linking step is less).
At present, as a conductive carbon black used in these semiconductive layers, acetylene black and furnace black are predominant.
For the semiconductive layer to secure necessary electroconductive level, it is generally required to contain such conductive carbon black in a large amount of 40 to 70 parts by weight per 100 parts by weight of the resin. Such a large amount of the conductive carbon black tends to deteriorate physical properties of raw materials used and extrusion moldability of the composition. As the base resin, therefore, ethylene/vinyl acetate copolymer (EVA) or ethylene/ethyl acrylate copolymer (EEA) which has a high comonomer content is mainly used. In other words, the base resin is required to be a copolymer which is allowed to have an increased comonomer content and acquire pliability and flexibility.
In the case of the semiconductive resin composition described above, since the base resin has a high polar group content, the composition has a high water absorption property and tends to exert adverse effects upon the electric property of the cable. Further, the composition has its electric property as high temperatures rendered unstable because the temperatures of the base resin for fusion and crystallization are lowered. The intimate adhesiveness of the resin composition with the insulating coating is not necessarily sufficient, because this composition has a different thermal expansion coefficient from the insulative polyethylene and the composition produces a delicate difference of surface energy from polyethylene due to the effect of the polar group. Particularly, when EVA is used as the base resin, it has a disadvantage that it will undergo thermal decomposition with evolution of corrosive gas during the stage of cross-linking and induce various troubles.
In the case of carbon black, "KETJEN BLACK" (trade name, a product by AKZO Co.), which can give a high conductivity in a small amount, since it has a large amount of impurity metal residue (ash), the resin composition containing this carbon black has an extremely poor resistance to thermal aging and is not suitable for use over a long period of time. Of various kinds of carbon black, this carbon black ranks high in terms of the DBP-absorption number (determined in accordance with JIS K-6221 by allowing 15 g of a sample carbon black to absorb dibutyl phthalate (DBP), measuring the volume in ml or DBP absorbed in the sample, and converting the value measured to the amount per 100 g of carbon black; the larger the value, the more the electroconductivity excellent) and, therefore, is advantageous in terms of electroconductivity. It nevertheless, suffers from a poor compatibility with the resin and, as a result, tends to form large protuberances in the co-extrusion boundary to the insulating polyethylene and adversely affect the electric property of the power cables produced.